The virus does this because, unlike most microbes, Zika can pass from blood into the brain, where it infects and kills stem cells, having severe effects on developing brains.

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But this ability to infect brain stem cells may prove useful for fighting deadly brain cancers, many of which are caused by mutated stem cells.

Jeremy Rich at the University of California, San Diego, and his team have tested the Zika virus on glioblastoma, the most common kind of brain cancer. Glioblastoma is one of the most difficult cancers to treat – even after surgery and other therapies, it usually kills people within a year of diagnosis.

The team found that exposing samples of human glioblastoma tumours grown in a dish to the Zika virus destroyed the cancer stem cells. It is these stem cells that usually kill a person, as they can become resistant to all available treatments.

Living longer

When the team tested the virus on ordinary brain cells from adults without cancer, they found that it didn’t infect this tissue – which may explain why Zika rarely causes problems in adults.

Next, the team tested the virus on mice implanted with glioblastomas. Normally such mice would die within a month, but those injected with Zika lived longer, with four out of nine still alive at two months.

It is unclear how this would translate to people, says Rich, as the disease affects mice differently to humans.

The researchers have no plans to start testing Zika in people with brain cancer as they are concerned the virus could pass to pregnant women: a mosquito species that carries Zika is found in some parts of the US, and the virus can also be transmitted sexually. Instead, they plan to see if they can genetically modify the virus to be safer, but still work as a possible treatment for brain cancer.

But Harry Bulstrode at the University of Cambridge, whose team has also been investigating this approach, is considering a trial of unaltered Zika in the UK.

Bulstrode says the South American epidemic has shown that Zika infection is usually mild in adults, making it fairly safe for anyone who isn’t pregnant. It has occasionally been linked to a form of paralysis called Guillain-Barré syndrome, but this seems rare, he says.

Bulstrode also points out that transmission is unlikely in the UK as the mosquitoes that carry Zika – Aedes aegypti and Aedes albopictus – can’t survive in the country, and that most people who get glioblastoma are over 50, so the risks of passing it on to a pregnant woman through sex are low.

While the virus is unlikely to lengthen the lives of people with glioblastoma by much, the small chance of benefit is worth investigating, says Bulstrode.

“This is an area of utmost need – we are talking about a uniformly fatal disease,” he says. “If it improves survival at all that would be an enormous result.”